Field of the Invention
The present invention relates to a process for producing anhydrous sodium carbonate. More particularly, it relates to a process for producing anhydrous sodium carbonate readily in high yield and with minimum consumption of heat energy by using, as a starting material, a crude sodium bicarbonate obtained by a so-called ammonium chloride-soda coproduction method wherein ammonium chloride and sodium bicarbonate are precipitated alternately, or by a so-called ammonia-soda method wherein only sodium bicarbonate is precipitated and ammonium chloride is subjected to distillation to recover and recycle ammonia.
Anhydrous sodium carbonate is widely used as a material for various industrial reagents or pharmaceuticals.
Reflecting the low level of economical development in recent years, the necessity for the conservation of fuel materials, particularly for the saving of energy, is emphasized in the industrial fields. Under such circumstances, it is required also in this particular field to cut down the costs as far as possible to improve the profit margin. For such a purpose, it is effective to improve the yield and to save energy. From such a viewpoint, there has been proposed, for instance, a method wherein sodium bicarbonate and/or sodium sesquicarbonate is suspended in a highly concentrated sodium carbonate solution to obtain a slurry having a NaHCO.sub.3 concentration of from 350 to 600 g per liter, and the slurry is counter-currently contacted with steam at a temperature of at least 150.degree. C. under bottom pressure of from 5 to 12 kg/cm.sup.2 and top pressure of from 1 to 10 kg/cm.sup.2 and decomposed in a single step to obtain anhydrous sodium carbonate in a suspended state (Japanese Examined Patent Publication No. 2652/1982).
This method is effective to some extent to facilitate the pyrolysis of the crude sodium bicarbonate or sodium sesquicarbonate and thereby to improve the conversion to the anhydrous sodium carbonate. However, it has a drawback that it requires a great amount of steam or heat energy for e.g. the mixing of the slurry in the vertical direction, and no consideration is taken into account from the viewpoint of the conservation of energy.
The present inventors have conducted various researches with an aim to obtain anhydrous sodium carbonate from sodium bicarbonate in good yield and with minimum energy, and have found it possible to attain the object by employing a multi-step process wherein the step of the pyrolysis of sodium bicarbonate is controlled to form a specific complex salt.